This invention relates to a process for controlling the coke balance (mass balance on coke) as well as the size of coke particles within a fluidized bed in the range adapted for a continuous operation of a heavy residual oil cracking apparatus, wherein thermal cracking of heavy residual hydrocarbons from petroleum or coal e.g., crude oil, topped crude oil, fuel oil residue, vacuum residue, tar sand oil, pitch, asphaltene, etc., (hereinafter referred to as heavy residual oil), is performed at high temperatures under fluidization of coke particles and steam, and wherein the coke balance is maintained positively, i.e., the amount of coke formed is larger than that of coke lost in the apparatus. More particularly this invention relates to a process for controlling the coke balance as well as the size of coke particles in the system, which comprises classifying one portion of the coke particles withdrawn from the fluidized bed into coarse particles and fine particles by means of a pneumatic classifier, and after the size of said coarse particles has been reduced by combustion, returning them to the fluidized bed along with said fine particles.
In a fluidized bed hydrocarbon thermal cracking apparatus using coke particles as a heat carrier, it is of great importance to control the coke balance in the system as well as the size of coke particles within the fluidized bed in the ranges adapted for operation, but it is considerably difficult to do so. This is because the major portion of the coke formed under cracking adheres on the surfaces of the coke particles within the fluidized bed and, at the same time the coke particles reduce their size by gasification, powdering, etc. within the fluidized bed. The coke balance in the system becomes positive when the amount of the above described adhering coke exceeds the amount of the coke lost by gasification, powdering, etc., whereas it becomes negative when the former falls short of the latter. In either case, it is usual practice to control the coke balance in such a direction that it may approach zero as constantly as possible by any means. As one of the means there has been proposed a method in which the coke balance is maintained by carrying out concurrently the control of the rate of deposition of the carbonaceous material adhering on the surfaces of coke particles and the control of the rate of gasification and combustion of the adhering coke (Japanese Patent Publication No. 6,502/1971). On the other hand, it is known that the coke particles become coarser and coarser with time in the fluidized bed. For this reason, unless the particles whose size has increased are selectively reduced in size, the size distribution of the coke particles within the fluidized bed would not be able to be constantly maintained in the range adapted for operation. However, procedures such as withdrawal from the system of the particles whose size has increased and additional supply of fine particles from outside of the system are not only considerably troublesome in operation but also uneconomical. Processes for controlling by mechanical treating the size of the coarse particles withdrawn from the system are known in Japanese Patent Publication No. 9,136,/1956, etc.